This invention concerns apparatuses and a method for reducing residual solvent levels, especially following “extraction” of biomass. This is the extraction of flavors, fragrances or pharmaceutically active ingredients from materials of natural origin (these materials being referred to as “biomass” in the body of this text).
Examples of biomass materials include but are not limited to flavorsome or aromatic substances such as coriander, cloves, star anise, coffee, orange juice, fennel seeds, cumin, ginger and other kinds of bark, leaves, flowers, fruit, roots, rhizomes and seeds. Biomass may also be extracted in the form of biologically active substances such as pesticides and pharmaceutically active substances or precursors thereto, obtainable, e.g., from plant material, a cell culture or a fermentation broth.
There is growing technical and commercial interest in using near-critical solvents in such extraction processes. Examples of such solvents include liquified carbon dioxide or, of particular interest, a family of chlorine-free solvents based on organic hydrofluorocarbon (HFC) species.
By the term “hydrofluorocarbon” we are referring to materials which contain carbon, hydrogen and fluorine atoms only and which are thus chlorine-free.
Preferred hydrofluorocarbons are the hydrofluoroalkanes and particularly the C1-4 hydrofluoroalkanes. Suitable examples of C1-4 hydrofluoralkanes which may be used as solvents include, inter alia, trifluoromethane (R-23), fluoromethane (R-41), difluoromethane (R-32), pentafluoroethane (R-125), 1,1,1-trifluoroethane (R-143a), 1,1,2,2-tetrafluoroethane (R-134), 1,1,1,2-tetrafluoroethane (R-134a), 1,1-difluoroethane (R-152a), heptrafluoropropanes and particularly 1,1,1,2,3,3-heptafluoropropane (R-227ea), 1,1,1,2,3,3-hexafluoropropane (R-236ea), 1,1,1,2,2,3-hexafluoropropane (R-236cb), 1,1,1,3,3,3-hexafluoropropane (R-236fa), 1,1,1,3,3-pentafluoropropane (R-245fa), 1,1,2,2,3-pentafluoropropane (R-245ca), 1,1,1,2,3-pentafluoropropane (R-245eb), 1,1,2,3,3-pentafluoropropane (R-245ea) and 1,1,1,3,3-pentafluorobutane (R-365mfc). Mixtures of two or more hydrofluorocarbons may be used if desired.
R-134a, R-227ea, R-32, R-125, R-245ca and R-245fa are preferred.
An especially preferred hydrofluorocarbon for use in the present invention is 1,1,1,2-tetrafluoroethane (R-134a).
It is possible to carry out biomass extraction using other solvents, such as chlorofluorocarbons (“CFC's”) or hydrochlorofluorocarbons (“HCFC's”) and/or mixtures of solvents.
Known extraction processes using these solvents are normally carried out in closed-loop extraction equipment. A typical example 10 of such a system is shown schematically in FIG. 1.
In this typical system, liquefied solvent is allowed to percolate by gravity in downflow through a bed of biomass held in vessel 11. Thence it flows to evaporator 12 where the volatile solvent vapor is vaporized by heat exchange with a hot fluid. The vapor from evaporator 12 is then compressed by compressor 13. The compressed vapor is next fed to a condenser 14 where it is liquified by heat exchange with a cold fluid. The liquified solvent is then optionally collected in intermediate storage vessel (receiver) 15 or returned directly to the extraction vessel 1 to complete the circuit.
The extraction of flavors, fragrances or pharmaceutically active components from materials of natural origin using chlorine-free solvents based on HFC's and other solvents as noted is of growing technical and commercial interest. One reason for use of the closed loop extraction circuit of Figure is to avoid the undesirable release of HFC or other solvents to atmosphere.
Residual solvent levels in extracts to be used in foodstuffs are governed by legislation in many countries. The levels of residual solvents in extracts for other applications are generally less strictly defined. Even where legislation does not dictate a specific level of solvent residue, it is generally desirable to minimize the residue levels where economic and practicable to do so. In the prior art, residual solvent residues have been reduced through a combination of heating and evacuation, or in the case of CO2 solvents by relatively mild evacuation alone. Unfortunately the process of heating and evacuation often has a detrimental effect on the aroma of the finished extract, many of the volatile “top note” components having been removed along with the solvent.
One of the key benefits of using a solvent such as 1,1,1,2-tetrafluoroethane (R-134a) for extraction of materials of biological origin is its ability to capture the relatively volatile flavor and fragrance components. In order to maintain the high quality of the extracts obtained with R-134a, a method of reducing the residual solvent levels that does not impact significantly on the organoleptic behavior of the extract is required.